Field of the Invention
This invention relates to a gas mixing valve, and more particularly to valves useful in the medical ventilator art.
Gas mixing valves combine a plurality of different gases to produce a gaseous output having desired proportions of each constituent. Such valves are used in medical ventilators to mix air and oxygen into an enriched breathing gas for a patient. In this application it is important, although the mixing ratio may change for different patients or different situations, that once a particular ratio is set it remain relatively constant throughout the operating cycle of the machine. In normal practice, however, it has been found that the mixing accuracy is a function of the pressure differential between the gases entering and leaving the valve. In a typical arrangement the air and oxygen are processed through individual pressure regulators and are each presented to the valve at approximately equal pressures of from 10 to 50 psi, depending upon the application. The gases undergo a pressure drop during transit through the valve, and emerge in a mixed state at a pressure that can be considerably less than the entry pressure. During variations in the gas flow rate accompanying the patient's breathing cycle, the pressure of the gases leaving the regulators and presented to the valve remains substantially constant. The magnitude of the pressure drop through the valve, on the other hand, is highly dependent upon the flow rate, increasing as the flow rate increases and decreasing as the flow rate falls.
Due to tolerances in the accuracy of the pressure regulators, the pressure of the gases presented to the valve are generally unequal and may actually differ by 0.5 psi or so. At high flow rates the pressure differential across the valve is considerably greater than this tolerance differential, and deviations from the desired mixing ratio are relatively minor. At low rates, however, the pressure differential across the valve approaches and may even fall below the tolerance differential. As a result the actual mixing ratio may vary widely from the theoretical ratio, and in the extreme the flow of one of the gases may be shut off entirely. Although many presently available valves exhibit good mixing accuracy under high flow rates, their accuracy deteriorates under the described reduced flow rate conditions.